Juan Martin del Potro overcame a stubborn challenge from David Ferrer to clinch their semi-final 6-4, 6-4 and advance to a title showdown against Roberto Bautista Agut in the Auckland Classic on Friday.

Del Potro, who sealed the Auckland title in 2009, shone in the key moments and fought off six of the seven break points Ferrer forged in a tense clash, with two games in the second set lasting almost 10 minutes apiece.

The Argentine's massive forehand, however, was the major defining factor in the match as he worked himself into a position to drive winner after winner past the Spaniard and clinched the semi-final in one hour, 46 minutes.

"I'm exhausted," Del Potro said in a court-side interview. "We played a great match. I think the crowd enjoyed the game. I did too. It's never easy against David because he is such a fighter and runs a lot, I just took my chances."

Del Potro will enter the top 10 for the first time since August 2010 on Monday ahead of his Australian Open first round clash against American teenager Frances Tiafoe.

He won the Auckland tournament in 2009 before going on to win his maiden grand slam title at Flushing Meadows later the same year.

However, the tall Argentine has battled with serious wrist injuries since 2014 and had to rehabilitate from four surgeries, but showed that his forehand, especially the ability to clinch winners on the run, will be a big threat in Melbourne.

"It's amazing to me to be playing tennis again after my wrist problems," Del Potro added.

"I'm just so pleased to be able to make another final."

Earlier, fifth seed Bautista Agut outlasted Dutchman Robin Haase in an epic clash that took almost three hours before the Spaniard produced an audacious backhand lob that kissed the baseline to clinch the 6-7(7), 7-6(3), 7-6(5) win.

"Unfortunately the match can't have two winners," an exhausted Bautista Agut said after the two hour, 52 minute clash. "It was an unbelievable battle on the court.

"At the end everything was close, everybody knows we left everything on the court and anything could have happened."

The 29-year-old, who won the title in 2016 but missed his defence last year due to illness, sunk to one knee and let out a massive yell of jubilation after he saw the ball drop in.

"The end was unbelievable," he said. "When I saw the ball was in, I went crazy."

"I passed very bad moments on the court and that moment I screamed because I was very stressed."

"I'm just so happy that I have won."

Barty downs compatriot Gavrilova to reach Sydney final

Ashleigh Barty overcame a slow start to win her all-Australian semi-final against Daria Gavrilova 3-6, 6-4, 6-2 and advance to the final of the Sydney International.

The 21-year-old needed one hour, 58 minutes on Friday to see off the challenge of the Russian-born Gavrilova as both players struggled to hold serve throughout the match.

Barty held just once in the first set but reversed the momentum in the second when she broke in the ninth game after Gavrilova double faulted twice, which prompted the 23-year-old to smash her racquet in frustration and earn a code violation.

Barty then broke early in the decider and again in the seventh game to ease to victory and set up a final against either Camila Giorgi or Angelique Kerber.

"I certainly shot myself in the foot with my serve today," she said. "But happy to get through in the end and get another opportunity tomorrow."

"Its been a pleasure playing here this week."

Gavrilova was playing her third successive Australian opponent in Sydney.

She had already beaten Olivia Rogowska in the first round and former US Open champion Sam Stosur in the second and advanced to the semi-finals after Wimbledon champion Garbine Muguruza withdrew with a hip injury.

Champion Mertens reaches Hobart final with Watson win

Defending champion Elise Mertens of Belgium reached the Hobart International final with a 6-4, 1-6, 6-2 win over British qualifier Heather Watson on Friday.

The second-seeded Mertens recovered from a second-set lapse to edge out Watson, who won the tournament in 2015, in one hour and 42 minutes.

The 22-year-old world number 36 will play Mihaela Buzarnescu in Saturday's final after the Romanian sailed past fifth-seeded Ukrainian Lesia Tsurenko 6-2, 6-2 in the second semi-final.

"She made the same run (from qualifying) as I did last year, but I'm happy to win today," Mertens said in an on-court interview.

"I gave it all today, and I'm still playing doubles later."

"I tried to play a bit more aggressive in the third set and make her run a little bit more. She was dictating me in the second set, all credits to her. She played an amazing match."

After a tight opener which was decided by a single break of serve, Watson, ranked 74th in the world, fought back to square the match but her hopes of winning a fourth WTA title were dashed due to unforced errors.

"(It is) just frustrating. Thought I made too many unforced errors," Watson said. "Because I'd been playing so well, I expected more of myself."